1. Field of the Invention
The invention relates to a ballast for operating a high pressure gas discharge lamp at high frequencies, and in particular, to a ballast which detects arc instabilities and adjusts its operating frequency to avoid visible flicker during lamp operation.
2. Description of the Prior Art
High pressure discharge (HID) lamps, such as mercury vapor, metal halide and high pressure sodium lamps, are typically operated with a magnetic ballast at or slightly above normal power line frequencies, e.g. 60-100 Hz. It would be desirable to provide an electronic ballast which operates HID lamps at high frequencies at above about 20 kHz. High frequency ballasts are becoming increasingly popular for low pressure mercury vapor fluorescent lamps. The high frequency operation permits the magnetic elements of the ballast to be reduced greatly in size and weight as compared to a conventional low frequency magnetic ballast. High frequency operation also provide substantial increases in lamp efficacy on the order of 10-15% for fluorescent lamps because of reductions in cathode drop. Similar reduction in size and weight would be desirable for HID lamps, especially for lower wattage metal halide lamps used for shop and track lighting, because it would provide greater flexibility in designing aesthetically pleasing fixtures for such uses. Lamp efficacy would also increase a few percent, though not nearly as much as for fluorescent lamps.
A major obstacle to the use of high frequency electronic ballasts for HID lamps, however, is the acoustic resonances/arc instabilities which can occur at high frequency operation. Acoustic resonances, at the minimum, cause flicker of the arc which is very annoying to humans. In the worst case, acoustic resonance can cause the discharge arc to extinguish, or even worse, stay permanently deflected against and damage the wall of the discharge vessel, which will cause the discharge vessel to explode.
The frequencies at which acoustic resonance occurs depends on many factors, including the dimensions of the arc tube (i.e., length, diameter, end chamber shape, the presence or absence of a tubulation), the density of the gas fill, operating temperature and lamp orientation. For high frequency ballasts, the operating frequency of the lamp current f.sub.I will generally be selected to be above the audio range (f.sub.I &gt;20 kHz), but may be lower. For the typical ballast operating with (distorted) sine waves, the power frequency f.sub.P is twice the frequency of the current, so f.sub.P will be greater than 40 kHz. The arc tubes, or discharge vessels, of high pressure sodium lamps and some of the newer metal halide lamps are ceramic and cylindrical in shape. The arc tubes of mercury vapor and metal halide lamps are made of quartz glass, typically with a cylindrical body and rounded end chambers. The power frequencies at which longitudinal acoustic resonance occurs for these generally cylindrical arc tubes can be approximated from the formula: ##EQU1## where L stands for the typical length of the arc tube, n denotes an integer number, c.sub.L denotes an averaged speed of sound in the length direction of the burner and equals approximately 450 m/s. The radial-azimuthal modes are given by: ##EQU2## where C.sub.r denotes an averaged speed of sound in the radial direction, R denotes the typical radius of the arc tube, and .alpha..sub.lm denotes the zeros of the first derivative of the Bessel functions.
The complete resonance spectrum f.sub.lmn is calculated from: ##EQU3## If the length of the arc tube is substantially larger than the radius, the frequencies at which flicker occurs can be estimated from formula (1) for longitudinal resonant frequencies.
For the specific case of a 100 W metal halide lamp with an arc tube length of 15 mm, for example, the lowest longitudinal resonant frequencies are expected to occur at power frequencies of 15 KHz. Therefore, higher order resonances will occur at power frequencies f.sub.P above 40 KHz, which correspond to current frequencies f.sub.I above the audible range.
Thus, the resonant frequencies can be approximated by calculation and/or observed through experiments by operating the lamps at varying frequencies and visually observing the resulting flicker. For a specific lamp type under specific operating conditions, an operating frequency can be selected at which visible flicker does not occur and a ballast designed to operate the lamp at this pre-selected frequency. However, the ballast would be limited to a specific wattage lamp of a specific manufacturer. Furthermore, changing operating conditions, such as changing environmental conditions or lamp blackening over life, which would alter the operating temperature and/or pressure, could change the resonant frequencies so that resonance occurs at the pre-selected ballast operating frequency. Alternatively, especially in the case of quartz glass arc tubes where dimensional control is difficult, even lamps from the same manufacturer would have different resonant points so that it is possible that a considerable percentage of lamps would flicker at the selected ballast operating frequency. Besides not being fault free, manufacturing a ballast for a specific lamp of a certain manufacturer is expensive in view of its limited market and is inflexible for the user. Accordingly, it would be desirable to provide a ballast for a broader range of lamps which senses arc instabilities during operation and selects the operating frequency to avoid arc instabilities due to acoustic resonance.
The article "An Autotracking System For Stable Hf Operation of HID Lamps", F. Bernitz, Symp. Light Sources, Karlsruhe 1986, discloses a controller which continuously varies the lamp operating frequency about a center frequency over a sweep range. The sweep frequency is the frequency at which the operating frequency is repeated through the sweep range. The controller senses lamp voltage to evaluate arc instabilities. A control signal is derived from the sensed lamp voltage to vary the sweep frequency between 100 Hz and some KHz to achieve stable operation. However, this system has never been commercialized.
JP 4-277495 (Kamaya) discloses a ballast which senses the impedance of the discharge lamp. If the impedance of the lamp is below a specified level, the ballast reduces high frequency oscillating components in the lamp current. A disadvantage of this design, however, is that the specified level is fixed, and as noted previously, the resonant frequencies in reality vary from lamp to lamp. Additionally, even though the high frequency components are reduced in the lamp current, there is no guarantee that operation will not shift to another resonant frequency at which arc instabilities occur.
Accordingly, it is an object of the invention to provide a method of detecting arc instabilities in gas discharge lamps, which is universally applicable regardless of lamp power, type, dimension, or physical or chemical composition. It is another object to provide such a method which may be implemented in a wide range of ballast topologies.
It is yet another object to provide a method of operating HID lamps at high frequencies to detect and avoid frequencies at which acoustic resonance occurs for a broad range of lamps, or at least lamps.
It is still another object to provide a lamp controller, or ballast, which implements this method.